The Home Office's Olympic Safety and Security Strategic Risk Assessment
document, published earlier this month, states that "as a high profile
event, the Games are likely to present an appealing target to individuals or
terrorist groups".

On track: David Cameron is hosting a Cabinet meeting on the Olympic Park to mark the 200 days to go countdownPhoto: EPA

MI5 is known to have identified four main categories of threat to the games. We outline them below.

1. Al-Qaeda

The first, and potentially most deadly, would be a plot masterminded overseas by al-Qaeda. The group’s leadership is known to want to repeat the carnage brought about by the London bombings of July 2005, although its capacity to do so has been undermined by the killing of Osama bin Laden and his putative successor Anwar al-Awlaki. A security report prepared for the Olympics states: “Historical evidence suggests that, of the different malicious attacks considered, conventional attacks on open transport systems, such as the Underground, are some of the more likely to be attempted.”

2. Lone Wolf

The second category, and the one security experts judge to be the most likely, is an attack by a 'lone wolf’ inspired by violent Islamism or other extremist ideologies. Although such individuals do not benefit from the planning and operational ability of larger terror cells, they are notoriously difficult for the authorities to infiltrate, monitor, detect or intercept.

Following the 7/7 bombings Britain suffered a number of attempted attacks by lone figures, such as Andrew Ibrahim, a former public schoolboy who converted to Islam and was found guilty in July 2009 of planning a terrorist attack on a shopping centre in Bristol using a homemade suicide vest

3. External dispute

Another, equally unpredictable, category of threat would come as a result of an international dispute between foreign groups or powers, such as Kashmiri or Tamil separatists. This kind of attack has been one of the games’ worst fears since Palestinian gunmen killed 11 Israeli athletes at the 1972 Munich games. One Whitehall official said: “The fear here is that we end up with a terrorist attack that is the result of a political dispute in which we are not involved.”

4. Republican dissidents

A fourth threat is posed by dissident Irish republican groups. MI5 recently highlighted its concern over the threat of an attack on the British mainland by such groups. What might hold such groups back is the international outrage such an attack would provoke.